1. Field of the Invention
The present invention relates to a method of forming a solder mask on a printed circuit board.
2. Description of the Prior Art
On a printed circuit board, a solder mask is formed on conductor circuits, for ensuring insulation between the conductor circuits for protecting the conductor circuits from peeling or oxidation, or for preventing unnecessary solder from sticking on the conductor circuits at the time of soldering operation.
As a method of forming this solder mask, screen printing process is typically well-known. However, the screen printing process provides substantially low resolution. It develops such phenomena as blur, pinholes, bleed, spread or the like, resulting in failure to deal with diminishing distances between conductor circuits or increasing densities of the printed circuit board that has taken place concomitant with the recent high density mounting of circuits.
Under this circumstances a method of forming the pattern by the photographic process, after forming a coating by applying a photosensitive resin composition by spraying or by use of a roll coater or curtain coater and so forth, has begun to be adopted (Japanese Patent Laid-Open Publication Nos. Sho 63-286841, Hei 2-28651 and Hei 2-77749).
However, in the above technique, since a liquid photosensitive resin composition having a relatively low viscosity is used as the paint for forming the solder mask, the thickness of the solder mask is thin, before the liquid has been dried on the circuit part and the circuit edge part. On this account, a solder mask thickness enough to have proper electric insulation, heat resistance and resistance to organic solvents, etc., can not be obtained. If the substrate has any through-holes, the photosensitive composition flows into these through holes and gives rise to various problems.
In order not to arise coating defects, such as sagging and flowing even when thickly coated, there has been proposed an electrostatic powder coating method using as an etching resist for forming printed circuits (Japanese Patent Laid-Open Publication No. Sho 63-246890), although outside the field of solder mask. However, in this method, a powder coating composition is coated on a conductive substrate of which one side is overly covered with copper. It is therefore difficult to apply on the printed circuit board in which insulating portions and electroconductive portions are mixed, because this printed circuit board is essentially insulated.
Also, the resist which is described in Japanese Laid-Open Publication Sho-63-246890 has poor photosensitivity to photo-initiated radical reaction and therefore difficult to be used as solder mask, because the powder coating composition suggested in this publication only contains solid thermoplastic resin having 0.5 to 5 polymerizable unsaturated groups in a molecular weight of 1,000. Since the crosslinking reactions in the powder coating composition only proceed through the polymerizable unsaturated groups, heat resistance, chemical resistance and durability are so poor that it is difficult to apply it to the solder mask.